While not usually threatening to plants, leafminor control is often necessary to manage the highly visible tunnels in leaves that can reduce crop value.
Popular in greenhouses, home gardens and landscaped areas across the country, leafminers are the larval (maggot) stage of many insects that feed between the upper and lower surfaces of leaves. On heavily infested plants it is not uncommon to find 6 or more maggots per leaf. Although damage can restrict plant growth, resulting in reduced yields and loss of vigor, healthy plants can tolerate considerable injury. Host plants include beans, blackberries, cabbage, lettuce, peppers, and a variety of ornamental flowers, trees and shrubs.
Adults (1/10 inch long) are often black to gray flies with yellow stripes and clear wings. They are similar in appearance to small, hunched-back house flies and lay their eggs on the undersides of leaves. Larvae are worm-like maggots (1/3 inch) which are often pale yellow or green in color. They create winding tunnels that are clear, except for the trail of black fecal material (frass) left behind as they feed.
Note: In some cases, pathogenic fungi and bacteria may enter old mines causing leaves to turn yellow and drop.
Mature larvae overwinter in the soil under host plants. As temperatures warm in the spring larvae pass to the pupal stage and appear as young adults in late April. Mated females use their needle-like ovipositor to lay up to 250 eggs just under the surface of the leaf epidermis. Deposited eggs may appear as small raised spots on the leaf. Within 10 days hatching larvae tunnel through the mid-leaf tissue, feeding as they go and leaving tell-tale wavy lines that are visible on the surface. Larvae mature in 2-3 weeks, and when ready to pupate, leave the leaf and drop to the soil. Once on the ground, they dig 1-2 inches into the soil and pupate. Adults emerge within 15 days as adult flies. There are several generations per year.
Pick off and destroy infested leaves in small growing areas. Maintain plant health with organic fertilizers and proper watering to allow plants to outgrow and tolerate pest damage. The parasitic wasp Diglyphus isaea is a commercially available beneficial insect that will kill leafminer larva in the mine. Use yellow sticky traps to catch egg laying adults. Cover soil under infested plants with plastic mulches to prevent larvae from reaching the ground and pupating. Organic neem oil will break the pests’ life-cycle by preventing larva from reaching maturity. Neem oil may also have repellent qualities and interfere with egg laying activities. Botanical insecticides can be used to knock down adult insects but have little effect on the protected larval stage feeding inside the leaf.
Note: Pest outbreaks often occur after general pesticide applications. This is because many of the pests natural enemies are affected by the pesticide.